How to trace origin of autoit

Good day,
I happen to find myself some virus like script named trojan-dropper.win32.autoit

From what I've found on the net, it's a script written on autoit. The problem is that my computer is not infected by it, however every couple of hours my antivirus finds some infected files on my external hdd like calculator.exe and others which he deletes.

So my question is how can I trace the origin of the script that creates these files, and how to trace the origin of the place this script starts from.


Thanks is advance.